The present invention is directed to hydrogen passivated heteroepitaxial III-V photovoltaic devices grown on lattice-mismatched substrates such as Ge and Si.
The present invention is further directed to the development of a hydrogen passivation process to significantly reduce or eliminate the electrical activity of dislocations in heteroepitaxial III-V materials such as InP grown on group IV substrates such as Ge and Si. This process may be incorporated into a solar cell fabrication process and result in the improved efficiency of such cells. The process may also be useful in the integration of high speed electronic and photonic devices.
Materials such as InP and its permutations grown on substrates such as Si, Ge and GaAs are of particular interest for large area, lightweight and mechanically strong solar cells for space applications. A key problem with these and other existing hetereopitaxial III-V photovoltaic cells is the presence of threading dislocations within the active regions of the cell. The dislocations form as a result of growing the III-V materials on lattice-mismatched substrates. For example, the large lattice mismatch between InP and certain group IV substrate materials (4% for Ge and GaAs, 8% for Si) result in the generation of high densities (on the order of 10.sup.8 cm.sup.-2) of threading dislocations within the InP bulk. Such dislocations degrade the electrical properties of the material, causing poor photovoltaic performance of subsequent heteroepitaxial solar cells. Specifically, dislocations can degrade cell characteristics in a number of ways by introducing shunting paths across photovoltaic p-n junctions, by generating recombination centers within the depletion region by generating traps within the quasineutral bulk, and by developing a high concentration of deep levels that can severely reduce minority carrier lifetime.
Prior efforts to reduce or eliminate the negative effects of dislocations in heteroepitaxial III-V solar cells consisted of methods aimed at reducing the total number of dislocations present, rather than addressing the electrical activity of dislocations. These prior techniques include the development of compositionally graded buffer layers, strained layer superlattice buffer layers, thermally cycled growth methods, and others.
These prior techniques have limitations. The magnitude of the lattice mismatch (i.e. the magnitude of the misfit strain in III-V materials grown on Group IV substrates) is large, yet high quality strain-relaxed III-V films over large areas are required for solar cell array feasibility. It is not possible to prevent rampant relaxation over such large areas at the temperatures required for growing these materials by conventional techniques. The net result is that threading dislocation densities will most likely be unable to reach the values necessary to achieve high efficiency cells using the prior techniques alone.
The present invention contemplates a hydrogen passivation process which overcomes the limitations of the prior art techniques. Instead of reducing the number of dislocations, the hydrogen passivation process described herein addresses the electrical activity of the cell dislocations.